These beautifully executed guides, part of the series Crinkleroot's 100 Animals Every Child Should Know, aim to ``provide a base of knowledge of the animal kingdom,'' Arnosky writes. In the persona of jolly Crinkleroot, a bearded and buckskinned mountain man, the author begins each book amiably with a letter's worth of general information concerning habitats, swimming or flying, gills or feathers. Precise yet fluid watercolor paintings of commonly known species follow--two or three per spread. Arnosky's choice of a sunlit palette against an expansive white background pleases the eye while helping to train it to particulars. His sometimes scientific, sometimes intuitive pairing of varieties breaks the predictable rhythm of many guide books: the heron and stork are coupled for their similarities of habitat and appearance, while the angelfish, flying fish and sea horse have been grouped because of their exotic shapes. As pleasurable as these books are, their overriding value may be in their adaptability to the teaching of observation skills: with just a little help from adults, youngsters may learn how to differentiate shapes, lines and functions, and to draw intelligent conclusions. Of the two, perhaps 25 Birds is more appropriate for younger readers, because of its subjects' greater accessibility in the child's natural world. Ages 2-8. (Aug.)